1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly relates to an image forming apparatus that forms a toner image.
2. Description of Related Art
As a conventional image forming apparatus, there is known, for example, a digital image forming apparatus described in Japanese Patent Application Laid-Open No. H08-251364. The digital image forming apparatus performs image concentration stabilizing control, which is described below. Specifically, a predetermined test toner image is formed on a photoreceptor by use of a developing unit. A toner adhering amount of the formed test toner image is detected by an AIDC sensor. A printer control section decides a grid potential of a charger and a development bias potential of each developing unit based upon the detected toner adhering amount, to form a predetermined image by use of the decided grid potential and development bias potential. Further, the printer control section changes an image forming condition for a next test toner image based upon the detected toner adhering amount such that the AIDC sensor can perform detection in an area with its detection sensitivity being high. It is thereby possible to accurately detect a toner adhering amount of the test toner image without being affected by characteristic fluctuations of the photoreceptor and a developer, so as to constantly form a favorable image by use of an optimal image forming parameter.
However, the digital image forming apparatus described in Japanese Patent Application Laid-Open No H08-251364 has a problem of a decreased print rate. More specifically, when the number of sheets of printed paper, operation time and the like satisfy certain conditions, the digital image forming apparatus executes the image concentration stabilizing control even without the need for changing the grid potential and the bias potential before or after the execution of the image concentration stabilizing control. Therefore, when certain conditions are satisfied during a printing operation, the digital image forming apparatus interrupts the printing operation and performs the image concentration stabilizing control even if there is no need for changing the grid potential and the bias potential. Such interruption of the printing operation causes a decrease in print rate in the digital image forming apparatus.